I thought I'd elaborate a bit in the zero volume topic since it's a way to provide more insight into design issues with LDRs and some product changes coming up.
To achieve absolute zero volume at step 0/mute the series LDR should be totally off (dark - mega ohms). No problem. Meanwhile the shunt LDR needs to be at minimal resistance thus essentially putting audio output at or near ground level, i.e. zero sound.
To achieve zero output the shunt LDR needs to be at fully cranked up which can stress the LDR thus encouraging it to drift and possibly shorten it's useful life if left in this state indefinitely. Moreover, the resistance of the shunt LDR will never be actually zero; more like 50 ohms at best. No harm done if driven to this state briefly during temporary step 0/mute. Where this becomes an issue is when the LDR preamp is "off" while powered up. Being a passive, the "off" setting for an LDR preamp should be the same as step 0/mute.
In order to preserve the useful life of the shunt LDR, we don't really want to keep the shunt LDR at max energy level. Unfortunately, backing off to some intermediate level, while being kinder to the LDR, may let audio to leak through if a connected source is still delivering live audio. Completely removing power to the LDR preamp will exacerbate this since without power the shunt LDR goes to max resistance and thus audio output floats and can make it even more likely to leak live audio through from the input.
Kind of makes you pine for the simplicity of the lowly potentiometer. Kind of.
A practical solution to this is to add a simple relay between the audio output and ground. Ordinarily this connection is open. But at step 0/mute or when the preamp is off, this relay fully grounds the output thus fully ensuring zero sound while also allowing the shunt LDR to be turned off or at least dialed way back from min resistance.
We are releasing a new version of our IO3 relay switching board that includes these very output-to-ground relays to help address this. Please note that audio does not pass through these relays, it only gets shunted to ground when the preamp is muted or off.
Cheers,
Morten
